At the end you'll notice Marshall says that the main problem with implementing tail recursion in REBOL was that he was the only person in the whole company who completely understood the process and could extend the interpreter since it involved studying the denotational semantics of REBOL and making evaluation to be explicitly CPS so you can eta-reduce all the continutations and remove any identity combinators.

I figured more people at a company that makes and sells a programming language would be familiar with the arcanity of programming language implementation.